(a) Field of the Invention
This invention relates to a screw member such as a bolt or a nut, for use in joining together various component parts in electric appliances. More particularly, it is concerned with a multi-headed screw member having a thin neck portion (or portions) to be sheared with a predetermined tightening torque.
Whether the screw is tightened with a predetermined torque or not, is verified in general by use of a torque wrench. However, if a number of the screws are used, it is often very difficult to check and verify the tightening torque of each and every bolt or nut. Moreover, failure occurs sometimes in such verification.
Also, when electrically conductive members in an electric appliance are to be joined together by the screws, if the tightening force is insufficient, the conductors generate heat due to insufficient contact between pairs of them so as to be liable to cause unexpected troubles and disorders. On the other hand, if the tightening force is excessive, there is a possibility of the screws and connected parts of the conductors being damaged.
(b) Description of Prior Art
In view of the problems described above, various sorts of screw members have been proposed, capable of exerting a predetermined tightening force without use of the torque wrench.
Such screw members have head parts divided into an inner head part and an outer head part by a thin neck portion to be sheared at a predetermined tightening torque. The thin neck portion is defined at approximately the middle portion of the screw member head where a screw member driving tool such as wrench, spanner, etc. is applied, (e.g., to the head 1C of a bolt 1 as shown in FIG. 1 of the accompanying drawing).
The double-headed bolt 1 shown in FIG. 1 is tightened by applying a tool on the outer head part 1C. When the tightening torque reaches a predetermined value, the thin neck portion 2 breaks (shears), and the tightening operation is complete. Further tightening or loosening of the already tightened bolt can be done by applying the screw driving tool to the inner head part 1A which remains after the outer head part 1C is twisted off, and turning it.
As stated in the foregoing, the bolt 1 should always be turned by applying the screw driving tool on and around the outer head part 1C. However, in case many bolts 1 are being assembled, or the work is done in a hurry, it sometimes happens that the tightening tool is mistakenly applied to the inner head part 1A and turned. In such case, since the thin neck portion 2 is not sheared, no determination can be made as to whether or not a predetermined tightening torque has been attained and it may happen that the screw is subjected to excessive tightening.
In other occasions, after the screw driving tool is applied to the outer head part 1C and the thin neck portion 2 is accurately sheared at a predetermined tightening torque, when an ordinary screw driving tool having no torque control function is applied on the remaining inner head part 1A to loosen the bolt 1 and re-tighten the same for re-assembling the electric appliance, etc. due to erroneous assembly, the fact that the bolt has been re-tightened cannot be recognized visually, with the result that the effect of controlling the tightening torque, which the bolt of this sort aims at, is negated.
Also, as in the case of a bolt 1 shown in FIG. 2, some of these bolts have triple heads, i.e., an inner head part 1A, a middle head part 1B, and an outer head part 1C divided by a large diameter neck portion 2A and a small diameter neck portion 2B, the two necks being sheared with differing tightening torque.
These triple-headed bolts, or nuts, as the case may be, are tightened by applying a screw driving tool to the outer head part 1C thereof, and then turning the tool. When the tightening torque reaches a first predetermined value, the small diameter neck portion 2B is sheared. Subsequently, the screw driving tool is applied to the middle head part 1B thereof, and then the tool is turned. When the tightening torque reaches a second predetermined value, the large diameter neck portion 2A is sheared, whereby the tightening work is completed. With this type of screw member, there can be performed two-stage tightening, i.e., so-called provisional tightening and final tightening.
Even in this case, there is the same possible mistreatment following initial installation as is the case with the double-headed bolt. In the case of the nut, either double or triple-headed, there is also the possibility that the nut will be inverted accidentally, negating the twist-off function entirely.